Plumbing and Gas,for better optical cryostats

Warren JohnsonLSU

Motivation for cryogenic interferometers

Higher mechanical Q (especially for metals)Lower Temperature (noise)Creaking and creeping frozen out? Dimensional

stability. Gas and dust frozen out. Every material (?)

becomes vacuum qualified.Superconductivity (ultra stable magnetic fields)

Practical Price?

• Extra layers in vacuum system – 77-K shell– Thermal radiation shields– X-K shell, where 4K < X < 300K

• One added vacuum feedthrough layer for electrical wiring.

• Three gas-liquid handling systems (LN, LHe, X )

Fundamental Price

• New mechanical design issues– Thermal contraction is large, and material

dependent. – Thermal conductivity of can have huge changes.

• New electrical design issues – Electrical conductivity can have huge changes

• New optical design issues– Optical absorption in bulk and ?

Test Mass Material

• Fused silica is disqualified (mechanical Q goes bad fast at lower temperature)

• Single crystal sapphire is limited by relatively high optical absorption

• Single crystal Silicon appears superior is almost everyway. [Industry wins again !] – except for need to change lambda to >1.5 micron.

Lessons learned with cryogenic bar detectors

• Handle the practicalities with good engineering (we physicists are unnaturally good at fooling themselves). Reliable and low maintenance cryogenics can be bought.

• Need many trials of each new feature. That’s why they call it research. Run cycle must be short, changes and measurements frequent.

Lessons …

• Low temperature can allow some tremendous technical improvements.

Normal cryostats are a problem

• Experimental chamber (X shell) must be raised and lowered during each run

• need a different configuration

Want cryogenic bell jar configuration

• A a fixed cold platform or table for construction of optical layouts, suspensions etc. Platform in ‘rigid’ contact with floor.

• 2 pi of accessgas heat exchanger on underside of platform77K heat exchange below that

• That space enclosed (by rings) and a jar.– Optical, electrical and gas feedthroughs through

rings

Cont…

• All isolation and suspension in the cold X space. • X space gas pressure easily variable between 1e-6 Torr and 1 TorrStandard thermal control of platform. (heaters,Thermometers.

Jena cryostat has most (or all?) of these features.

• Cryogenics should be as easy as vacuum.– Specify the size, buy from vendors, work the

controls.